Field of the Invention
The present invention relates to a machine tool capable of increasing the tool life by causing a machining program for tool center point control at a certain tool orientation to operate and performing machining while changing the tool orientation based on a set tool use range and tool orientation change waveform pattern.
Description of the Related Art
A machining center provided with a main spindle fixed on a linear movement axis or a rotation axis, a table, for fixing a workpiece, having one or more linear movement axes or rotation axes, and a numerical controller for controlling the main spindle and the linear movement axis and the rotation axis of the table is known.
Generally, a turbine blade, which is one of turbine components used for an aircraft or an electric generator, is machined by a machining center having three linear movement axes and at least one rotation axis. As shown in FIG. 1A showing a turbine blade shape and FIG. 1B showing a machining path, in the case of machining based on simultaneous control of the linear movement axes and the rotation axis by a micro linear instruction, as in the case of semi-finishing machining or a finishing machining in contour machining of the turbine blade or the like (machining shape 1), helical machining is performed along the machining path shown by a machining path 3 by using a side surface of a ball end mill (a tool 2) at a certain tool orientation with respect to a workpiece machined surface. Such a machining method generally uses a tool center point control function.
The machining method described above is not for turbine blades alone, and is a general machining method used in contour machining using a linear movement axis and a rotation axis. In the machining method as shown in FIGS. 1A and 1B, a method is often adopted, by CAM software, of creating a machining program by determining, with respect to a certain axis, a certain surface or a certain machined surface, a tool orientation by taking, as shown in FIGS. 2A and 2B, an angle formed by a tool axis 4 and the movement direction of a tool (a tool movement direction 5) with respect to the tool movement direction 5 and a plane perpendicular to the tool movement direction 5 as a lead angle 6, and an angle formed by the tool axis 4 and a line segment of intersection of the surface of the machining shape 1 and a plane perpendicular to the tool movement direction 5 as a tilt angle 7.
However, as shown in FIG. 3, with this machining method, the same part of the blade of the tool machines the workpiece at all times, and wear of the tool progresses only at one part. The reference sign 2a indicates the part of a tool 2 where wear progresses due to use. Especially, since machining of a turbine blade or the like uses a metal called a heat-resistant alloy, such as a nickel-based alloy, with extremely poor machinability, wear of the tool progresses rapidly. Accordingly, machining conditions have to be lowered, or the tool has to be replaced. As a result, the tools may become very costly, and also, a step may be produced on the machined surface due to tool exchange and may result in product failure, and thus efficient machining becomes difficult.
Accordingly, as a method of performing more efficient and economic machining, a method disclosed in Japanese Patent Application Laid-Open No. 5-8148 is sometimes adopted. With this method, a machine tool capable at least of 3-axis control changes, during machining, the tool orientation of a tool whose blade edge is formed in an arc shape at the tip end with respect to a workpiece which is a machining target so as to uniformly use the entire cutting edge to thereby increase the tool life.
According to the machining method disclosed in Japanese Patent Application Laid-Open No. 5-8148, the cutting edge may be widely used and the tool life may be increased by a method of performing machining while changing the contact position of a ball end mill by a machining program for performing machining while changing the tool orientation as shown in FIG. 4.
However, as shown in FIG. 5, in the case where the main spindle rotation speed (rpm) and the feed speed (mm/min) are constant, with the method of widely using the cutting edge by changing the tool orientation, since the cutting speed (m/min) is different depending on the contact diameter of the blade edge of the tool 2, machined surfaces with different machining qualities are created. This may result in unstable machining of a heat-resistant alloy, especially, a nickel-based alloy.
With respect to a turbine blade whose main material is a nickel-based alloy and the like, it is important from the standpoint of its usage environment that the parts performance is not impaired under harsh conditions of high temperature and high pressure. Thus, the composition state of the machined surface where machining or the like is performed is strictly specified. The composition state varies greatly by the cutting speed (m/min) and the feed f (mm).
With the method as shown in FIG. 5 of widely using the cutting edge by changing the tool orientation, the cutting speed (m/min) varies greatly according to the contact diameter of the blade edge. In FIG. 5, for example, in the case of changing, for an R5.0 ball end mill, the tool orientation within the range of positions of 10° to 80° of the tool tip end, when the main spindle rotation speed (rpm) is 2000 (rpm), the cutting speed of the outer contact diameter at the 80° position is 62 (m/min), but the cutting speed at the 10° position is 11 (m/min). In this manner, since there is an about sixfold cutting speed difference depending on the tool orientation, there is a high possibility that the composition state of the machined surface is greatly affected, and a uniform machined surface quality is difficult to achieve.
As a method of achieving a uniform machined surface quality, a method disclosed in Japanese Patent Application Laid-Open No. 2002-96243 is sometimes adopted. This method is for performing continuous machining under an optimal machining condition where the cutting speed and the feed are constant, by attaching, to machining program data, the magnification between the distance from the contact point between a tool and a workpiece to the center of the tool and the radius of the tool and by performing re-calculation and re-instruction regarding the rotation speed and the feed speed by a numerical controller based on the magnification.
It is possible to achieve a uniform machined surface quality by a machining device that performs control of achieving a certain cutting speed (m/min) that is set for a tool diameter at a certain tool orientation by using the machining method disclosed in Japanese Patent Application Laid-Open No. 2002-96243. However, in the case where the tool 2 is a ball end mill, as shown in FIGS. 6A and 6B, due to its shape property, the radius is small and the main spindle rotation speed (rpm) is great at a position near the center of the tool. In FIGS. 6A and 6B, in the case of changing the tool orientation of a ball end mill with a radius of 5.0 within the range of positions of 10° to 80° of the tool tip end, if the main spindle rotation speed is 2000 (rpm) at the 80° position (outer contact diameter φ9.848) (FIG. 6A), the main spindle rotation speed is 11345 (rpm) at the 10° position (outer contact diameter φ1.736) (FIG. 6B).
When a big difference in the main spindle rotation speed (rpm) is caused depending on the tool orientation in this manner, there is a concern that a displacement difference based on increases in the temperature of the main spindle at the time of low rotation and at the time of high rotation affects the machining accuracy. Also, since the feed speed (mm/min) has to be increased or decreased in proportion to the main spindle rotation speed (rpm), the feed speed (mm/min) is made excessively great, and there is a possibility that the shaft movement may not follow due to the overspeed of the main spindle rotation speed or the excessive feed speed. To prevent the overspeed of the main spindle rotation speed or the excessive feed speed, the method described in Japanese Patent Application Laid-Open No. 2002-96243 may set the maximum rotation speed and the minimum rotation speed, but if the cutting speed is made constant, the tool use range will be limited, and this is not a realistic method.
As described above, a conventional machining device that controls a certain cutting speed (m/min) set while using a ball end mill is not practical for turbine blade machining.
Accordingly, in recent years, a barrel tool 8 as shown in FIG. 7 having a barrel radius R with a great radius of curvature at the tool side surface shape is sometimes used. Due to its shape property, this barrel tool 8 has a small difference between the minimum diameter and the maximum diameter at the tool tip end. As shown in FIGS. 8A to 8C, for example, in the case of using the range of φ6.0 to φ8.0 of a barrel tool 8 with φ8.0 whose radius of curvature is 100 mm, if the main spindle rotation speed at the position of φ8.0 is 2000 (rpm), the cutting speed of the outer contact surface φ8.0 is 50 (m/min), as shown in FIG. 8A. In the case of controlling a certain set cutting speed (m/min) as shown in FIG. 8B, the main spindle rotation speed is 2290 (rpm) at the position of φ7.0. In the case of controlling a certain set cutting speed (m/min) as shown in FIG. 8C, the main spindle rotation speed is 2667 (rpm) at the position of φ6.0.
By using such a barrel tool 8, the variation in the main spindle rotation speed (rpm) is reduced, and the displacement difference based on increases in the temperature of the main spindle at the time of low rotation and at the time of high rotation is reduced and the influence on the machining accuracy may be made small. Also, since the feed speed (mm/min) is increased or decreased in proportion to the main spindle rotation speed (rpm), a variation in the feed speed is also reduced, and the influence on the follow-up characteristic of the shaft movement is reduced.
However, in the case of machining the shape of a turbine blade, since the barrel tool 8 has a great radius of curvature on the tool side surface due to its shape property, the machining pitch (pick feed) by a conventional ball end mill is too small, and this may result in poor cutting due to insufficient biting of the tool blade edge resulting from an excessively small depth of cut. Thus, the machining pitch in the case of using the barrel tool 8 has to be increased compared with the machining pitch in the case of using a ball end mill. As a result, by increasing the machining pitch, the machining length is reduced and the machining time is reduced, but the depth of cut is increased by the increase in the machining range due to the large radius R of the barrel tool 8 and the large pitch, and the cutting heat that is generated is increased compared with the case of the ball end mill. In the case of machining a heat-resistant alloy, there is a problem that the cutting heat is accumulated in the tool and the tool life is reduced.
As a method for solving this problem, there is a method of creating a machining program by a CAD/CAM device for creating a machining program, by precisely specifying the machining orientation based on a machining position and re-calculating the machining program to thereby change the machining orientation. However, to create a machining program for changing the machining orientation, an expensive high-performance CAD/CAM device capable of defining a machining orientation by one machining instruction is necessary, and also an engineer with full knowledge of CAD/CAM operation is necessary. Moreover, even for an engineer with full knowledge of CAD/CAM operation, it is not easy to create a machining program in which the machining orientation is changed.